Hello Ossie,
So there is a coaxial Besa on AVRE Mk VIIs?
Reason I'm asking is, that e.g. in this picture
and also in the Tamiya book on the Churchill, there is a hole next to the gun, but it is in the same location as the gun sights, and the Besa 'cradle' that would normally be next to the gun, isn't there.
As for the 'bore evacuator', in Osprey's "The Churchill tank" there is a photo showing an AVRE Mk VII with something that looks like a large drum halfway round the barrel.
All the best,
Erik Jan